This invention pertains in general to testing apparatus and more specifically to apparatus for electromagnetic pulse (EMP) testing.
It is often desirable to test electronic equipment, systems and subsystems for susceptibility to severe electromagnetic environments. Likewise, it is desirable to determine the effectiveness of protective devices and electromagnetic radiation hardening measures.
Two approaches to EMP testing are typically used. EMP testing may be done through full illumination testing of the equipment under test in a large simulator. This is a traditional manner of EMP testing, but its limited availability and high cost makes it inappropriate for many users. A second approach which is typically more reasonably priced and versatile is to perform the testing on individual subsystems or line replaceable units.
In the second approach, high-level electrical stimulus is directly applied to individual elements of the system at predictable points of entry. In "Linear Amplifiers for Direct-Drive Testing" by Michael E. Gruchalla, ITEM 1990, R & B Enterprises, pp. 82-96 (1990), various testing arrangements are described. As pointed out in the Gruchalla article, the direct application of electrical stimulus or "direct-drive testing" has been made feasible for two reasons. First, by using direct-drive, the test levels required are much lower than those needed for total system illumination. Second, the historical record of full illumination provides a basis for predicting the electromagnetic stresses in specific applications.
Recent changes in military and commercial EMP test requirements have been developed. Specifically, MIL-STD- 461C added four tests, CS10, CS11, CS12 and CS13, to the existing military standard MIL-STD-461B. The tests set forth in these standards are similar to those of the new commercial standard DO-160C Section 22. In all these tests, it is required that a high energy pulse be applied to the unit under test to simulate lightening or other EMP. Certain of these testing arrangements attempt to simulate EMP coupling to cables and antennas. This technique may be referred to as direct-drive bulk injection.
Pulsed power sources generate EMP pulses in a test set-up. Two different approaches to provide a pulsed-powered source are generally used. The first and most common is the use of a tuned LC network. By charging a high energy capacitor and then discharging it into an inductor, pulses meeting the test parameters can be created. One limitation to this approach is that the frequency and decay rate of the resulting damp sinusoid waveform are fixed. Any change in waveform requirements results in a separate tuned high voltage circuit. Thus, this method can be very expensive where it is necessary to meet multiple test requirements.
A second approach utilizes a broad bandwidth linear RF amplifier driven by a signal generator capable of creating various waveforms within its bandwidth. The limitation of this technique is the relative higher cost of providing higher powered amplifiers as compared to the energy storage system.
In either test method, a coupling transformer is commonly utilized to couple the test signal to a cable. In the past, such transformers have typically been in the nature of isolation transformers with a 1:1 transform ratio.
Under the requirements of MIL-STD-461C, there are both voltage and current limits on the waveform to be applied to the device under test. Ideally, a generator with a known source impedance (typically 200 ohms) would drive the cable under test with the open circuit amplitude set at the voltage limit. Depending on the impedance of the cables, a corresponding voltage and current will be generated. Unfortunately, keeping a constant source impedance in the 10 KHz to 100 MHz frequency range is difficult, if not impossible. An alternative method is to determine which signal lines are high impedance and which are low impedance and then test them according to the voltage or current limit, whichever is most appropriate. This method will satisfy the test requirements without concern for the source impedance provided both the required voltage and current can be generated by the test equipment. Broadband amplifiers are typically considered to have a 50 ohm output impedance. However, they typically, in fact, vary from 10's of ohms at the low frequencies to 100's of ohms at the higher frequencies and change under different Load conditions. Achieving the current limit of MIL-STD-461C is not difficult due to the relatively low impedance of the amplifier and the ease of adding currents to the primary of the coupling transformer to step up the current. Developing the voltage is more difficult. In the past, a very high powered amplifier was required which was capable of directly generating the necessary voltage.